1. Field of the Invention
The present invention relates to a head position controlling device for tape recorders, more particularly, to a head position controlling device for tape recorders which employ the PCM recording system.
2. Description of the Prior Art
In recent years, the PCM recording system, replacing the conventional analog recording system, is being put into practical use for cassette tape recorders in order to realize recording and reproduction of high quality voice signals by securing a wide dynamic range without being affected by wow and flutter and by noise. The PCM recording system is a system which records voice signals on a magnetic tape by encoding the waveform according to whether there is a pulse or not, and reverts the codes pulses to the original voice signals at the time of reproduction.
In the PCM recording system, in order to make it possible to record and reproduce voice signals in the audio range of frequencies (between about 20 Hz and about 20 kHz), it is necessary in general to process information content of the order of several mega bits per second. For processing this much information in a short time, there are proposed several methods of recording and reproduction. One of such methods is the multi-track recording and reproducing system. This is a system in which the recording head is constructed in a stacked form in the lateral direction of a magnetic tape in order to make it possible to form a plurality of parallel recorded tracks in the direction of the magnetic tape. The reproducing head is also constructed, analogous to the recording head, in a stacked form in the lateral direction of the magnetic tape in order to correspond to the plurality of recorded tracks that are formed by the recording head. In this system, recording is made by means of the recording head by recording the encoded voice signals on each of the tracks, and reproduction is made by means of the reproducing head by reproducing the encoded voice signals on each of the tracks. Because of the circumstances described above, the width of the recording track is extremely narrow, somewhere between several tens of .mu.m and one hundred .mu.m.
Furthermore, in view of making the tape recorder small in size, making the units compatible in recording and reproduction, and the like, the recording head and the reproducing head are generally either of the type where the recording head serves also as the reproducing head or of the type where both heads are put into a single unit, the so-called combined head, moving in an interlocked manner.
In a casette tape recorder with the PCM recording system, it is apt to produce a relative slip in position between the magnetic tape and the head, due to outside vibrations acting on the magnetic tape and the head or to the accuracy of the running mechanism of the magnetic tape, or the like. Accordingly, at the time of recording and reproduction, the magnetic tape meanders relative to the head. However, the head runs off the recording tracks over which it should run under normal conditions and runs into the contiguous recording tracks, because the recording track has a very narrow width, as mentioned earlier, and the spacing between the neighboring recording tracks is less than the width of each track being of the order of several tens of .mu.m.
The servo tracking method is known as one of the countermeasures against the above problem. This is a method of forming a servo track at the time of recording and independent of the information tracks, and moving the head following the servo track at the time of reproduction in order to prevent the running-off of the head from the recorded tracks.
However, in the servo tracking method described above, the meandering of the magnetic tape that arises during recording will be recorded as is on the recording tracks and the servo track, since there are recorded on the magnetic tape no tracks that can be used as guide during recording.
Consequently, excessive load is placed on the tracking servo mechanism during reproduction, causing sometimes a run off of the head from the recorded tracks.
Furthermore, when a tape, which has already been recorded and reproduced on a recorder and has been taken out of the recorder, is subsequently placed in a recorder for reproduction, the positioning error is at least several tens of .mu.m so that it has been difficult to insure the reproducibility of the positioning accuracy of the corresponding positions between the magnetic tape and the head.
In addition, another countermeasure against the existing problem, that has been proposed is a method for improving the reproducibility of the relative positioning relationship between the tape and the head. According to the method, there is provided on the cassette or the recorder itself a guiding member which serves to restrict the running range of the magnetic tape by making direct contact with the edge of the magnetic tape at the intake side of the tape into the head. However, in a method of this kind, the edge of the magnetic tape in recording is forcibly position controlled by the guiding member so as to run on the head by contacting it at a prescribed relative position, so that there is a danger of creating buckling on the tape during recording due to the load acting on the tape. When buckling is formed, the meandering of the recording tracks becomes substantial so that an excess burden is placed on the tracking servo mechanism of the head. Also, even under reproduction with the servo tracking device there are drawbacks such as the reduction in the level of reproduced signals or even the impossibility of reproduction. Even when no buckling is formed during recording, the wear of the magnetic tape edges will increase under forced control on the running range of the magnetic tape in repeated reproductions such that either buckling apt to be formed or slips in relative positions are introduced after use over extended period, leading to a reduction in the reliability of the signal reproduction.